1. Field of the Invention
The present invention is directed relates to an electronic apparatus in which an optical engine (optical system) incorporates a liquid crystal display (hereinafter abbreviated as LCD) having a circuit that is constituted of thin-film transistors (hereinafter abbreviated as TFTs). In particular, the present invention relates to an electronic apparatus such as a projector that uses a transmission-type LCD.
2. Description of the Related Art
In recent years, the technique of forming TFTs on a substrate, such as a glass substrate, by using a semiconductor thin film, (typically a silicon thin film) has made rapid progress. Active matrix LCDs, that have a circuit using such TFTs as switching elements and that perform optical modulation by a liquid crystal, are now attracting much attention as next-generation display devices.
Among the electronic apparatuses now attracting a lot of attention and which use an active matrix LCD as a display medium, are transmission-type and reflection-type projectors. FIG. 3A outlines a basic optical engine of a three-panel transmission-type projector.
In FIG. 3A, reference numeral 301 denotes a lamp light source; 302 and 303 are dichroic mirrors that reflect only red light and green light, respectively; 304-306 are full-reflection mirrors; 307-309 are transmission-type LCDs corresponding to R (red), G (green), and B (blue), respectively; 310 is a dichroic prism; and 311 is a projection lens.
Strong light is emitted from the lamp light source 301, and only the R component beam is reflected by the dichroic mirror 302. The R component beam is changed in traveling direction by the full-reflection mirror 306 and enters the R-LCD 307.
Only the G component beam is reflected by the G dichroic mirror 303 and enters the G-LCD 308.
The B component light, that has passed through the dichroic mirrors 302 and 303, is reflected by the full-reflection mirrors 304 and 305 and enters the B-LCD 309.
Light beams carrying video information, that have passed through the R, G, and B-LCDs 307-309, are recombined by the dichroic prism 310 and projected onto a screen by the projection lens 311.
FIG. 3B is a schematic sectional view of each of the transmission-type LCDs 307-309. In FIG. 3B, reference numerals 312 and 313 denote pixel TFTs. A light beam 314 carrying video information that is applied from above is interrupted by black masks 315 and 316.
Since the number of carriers in a semiconductor increases when it is excited by light, it is necessary to prevent a light beam from being applied directly to the active layers of TFTs by using black masks as mentioned above. However, since a substrate 300 on which the TFTs 312 and 313 are formed is as thick as about 0.7-1.1 mm, part of the light beam 314 carrying video information is irregularly reflected inside the substrate 300 to cause stray light 317 and 318.
If stray light reaches an active layer, the number of carriers increases through optical excitation. The increase in the number of carriers results in leak current (off-current) that flows when the TFT is in an off-state. A large leak current may cause a display failure.